Radioactive tracer



Patented Get. 4, 1960 2,955,088 RADIOACTIVE TRACER Alan lBeerbower, Baltimore, Md., and William H. King, In, Flor-ham Park, N1, assignors to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed Mar. 8, 1956, Ser. No. 570,392

7 Claims. (Cl. 252301.1)

This invention relates to radioactive tracers and more particularly relates to radioactive tracers comprising radioactive gold. Still more particularly, the present invention relates to subdivided solids plated with gold 198, to their methods of preparation and to the uses of the radioactive gold-plated subdivided solids as radioactive tracers.

The importance of radioactive tracers is well known. Such materials have been used extensively, for example, in industrial, medical and agricultural research and have resulted in many instances in substantial improvements in their particular fields of application. The key to carrying out a successful tracing operation lies primarily in the proper selection of the radioactive tracer material which is to be employed in a given tracing operation. While a given radioactive tracer material may be outstanding in a particular tracer application, it may be, on the other hand, completely unsuccessful and inoperative in another tracing environment. There is thus a continuing need for the development of new and improved radioactive tracers and particularly for radioactive tracers which are adaptable for a variety of radioactive tracing operations.

A novel type of radioactive tracer has now been developed. More particularly, the present radioactive tracer comprises a subdivided solid plated with radioactive gold. A particularly useful radioactive tracer of the present invention comprises a finely divided solid plated with gold 198 such as, for example, a finely divided porous hydrocarbon conversion catalyst which has been plated with gold 198. The present radioactive tracers are prepared preferably by mixing a subdivided solid with a reducible compound of gold, reducing the reducible compound of gold to thereby plate the subdivided solid with gold metal, and then subjecting the gold-plated subdivided solid to the action of neutrons to thereby form radioactive gold. The present radioactive tracers are particularly useful, for example, for carrying out radioactive tracing operations in fluidized solids hydrocarbon conversion processes and for determining the efliciency of grease filtermg.

Any subdivided solid may be converted into a radioactive tracer material in accordance with the present invention. However, generally, the subdivided solids applicable to the present invention will have an average particle size diameter in the range of about to 2000 microns. The present invention is particularly applicable to finely divided solids having an average particle size diameter in the range of about 0 to 200 microns. Examples of finely divided solids include fluidizable hydrocarbon conversion catalysts (e.g. alumina, silica-alumina, etc), dirt or dust which may be found in grease, fine coke particles, diatornaceous earth, rock dust, etc. Larger subdivided solids may also be employed to prepare radioactive tracers in accordance with the present invention. For example, coarse hydrocarbon conversion catalysts having particle sizes in the range of about 200 to 500 microns and coarse coke particles having sizes from about 200 to 2,000 microns may be employed in the present invention. It will be understood that the present invention is not necessarily restricted to either one of these two general classes, namely, finely divided solids or coarse solids, but is applicable also to mixtures of such particles. For example, the coke particles in a fluid coking unit generally have particle sizes in the range of about 0 to 1,000 microns, the bulk of the particles having a size in the range of about 74 to 500 microns. Product coke from such coking units may have particle sizes in the range of about 0 to 2,000 microns.

The present invention is particularly applicable to porous, subdivided solids such as finely divided, porous hydrocarbon conversion catalysts. Such porous materials may be readily gold plated within the pores in accordance with the present invention such that attrition will not remove any substantial arnount of the radioactive gold plate. The surface area of porous, subdivided hydrocarbon conversion catalysts generally is in the range of about 50 to 600 square meters per gram, usually in the range of about to 300 square meters per gram. Pore sizes generally vary from about 20 to Angstroms, the usual range being from about 30 to 100 Angstroms.

Generally, the subdivided solids employed in the present invention will be inorganic. It is considered for the purposes of the present invention that coke particles, which are essentially pure carbon, are inorganic solids. However, it will be understood that the present invention is applicable also to subdivided organic solids such as subdivided polymeric materials, solid gels, etc. Preferably, the subdivided solid consists of materials comprising elements having low neutron capture cross-sections, that is, less than about 0.50 barns, preferably, less than about 0.25 barns. Examples of such low neutron capture crosssection materials are carbon, oxygen, silicon, aluminum, beryllium, phosphorus and fluorine.

In accordance with the present invention, the abovedescribed subdivided solids are gold plated. This may be done either with naturally-occurring gold (gold 197) or with gold comprising a radioactive gold isotope. Preferably, however, the subdivided solids are initially plated with naturally-occurring gold. It is particularly preferred to carry out the gold plating solely by chemical means. The chemical gold-plating method is generally most effective since it promotes the formation of a relatively uniform gold plate on the surface of the subdivided solids. More particularly, the chemical gold-plating method comprises mixing the subdivided solid with a reducible compound of gold and then reducing the reducible compound of gold to thereby plate the subdivided solid with gold metal. This particular gold-plating method is preferably carried out by mixing the subdivided solid with an aqueous solution of the reducible compound of gold. Gold chloride (especially auric chloride) is particularly preferred as the reducible compound of gold although other reducible compounds of gold can be employed such as, for example, auric bromide, cyanide, hydrogen nitrate or sulfate, or the mixed alkali-'turic salts. Aurous salts are less preferred as they are generally too water insoluble. Preferably, the amount of the aqueous solution which is mixed with the subdivided solid is sufficient to form a thin plate of gold on the subdivided solid. The particular proportions of subdivided solid and aqueous gold solution employed will depend upon the surface area of the subdivided solid. When a porous subdivided solid is employed, the amount of gold solution employed should be sufiicient to fill the pores and to leave a wet powder with solution adhering to the surface of the subdivided solid. Generally the amount of the gold compound employed should be suflicient to form a gold plate on the subdivided solid which amounts to about 0.001 to 1.0, usually about .01 to 0.5% by weight of gold based on subdivided solid plus gold.

gold-plated subdivided solid will be formed after about /2 to 2 hours, usually after about /4 to 1 hour. Preferably, a reducing agent is added to the mixture of the reducible compound of gold and the'subdivid'ed solid.

Generally, about 0.5 to 5%, preferably, about 1' to 2 weight percent of the reducing agent will be incorporated into the mixture based on the amount ofreducible gold compound. Specific examples of suitable reducing agents.

include oxalic acid, hydroquinone, p-amino-phenol, pyrogallol, catechol, sodium bisulfite, etc. i

The subdivided solid, plated as described above with naturally-occurring gold (gold 197), is then subjected to the action of slow or thermal neutrons to thereby convert at least a portion of the naturally-occurring gold to radioactive gold (gold 198). This can be most conveniently accomplished by irradiating the gold-plated subdivided solids in an atomic pile. Generally, the radiation in such atomic piles (or nuclear reactors) will comprise about to 10 usually about 10 to 10 slow neutrons/cm. /sec. Usualily slow neutrons are considered to be those neutrons which have an energy of less than about 30 electron volts. Fast neutrons are also present in these atomic piles. Generally, the fast neutrons will have an energy in the range of about 30 to 6x10 usually about 30 to 2 10 electron volts.

4 described above in detail will be employed for the irradiation and the gold plating.

The gold 198 plated subdivided solids of the present invention are particularly outstanding for use as radioactive tracers. More particularly, the present tracer materials have the following outstanding properties: (1) low vapor pressure," (2) chemical inertness, (3) insclubility, (4) strong radioactive emission, (5) short radioactive half life, and (6) ease of preparation. It was found that no other radioactive tracer material proposed heretofore met all of the above desirable requirements. Gold 198 has ahalf life of 2.7 "days and decays by. giving oif beta rays (0.97 million electron volts) .and gamma rays (0.41 million electron volts).

The present radioactive tracers are particularly useful for tracing operations in hydrocarbon conversion processes which are carried out'employing subdivided inorganic catalysts. The present invention is especially applicable to those hydrocarbon conversion processes which are carried out employing finely divided, porous solid catalysts such as, for example, aluminum silicate, silica gel, diatomaceous earth, fullers earth 'and similar ma- The fast neutron flux will generally be in the range of about 10 to 10 usually about 10 to 10 neutrons/ OHL2/S6C. In addition to the neutron flux in these atomic piles, there will generally be a gamma ray flux of about 10 to 6x10 usually about 10 to 3 l0 yroentgens per hour. The pressure and temperature conditions during the irradiation are not particularly critical. However, the irradiation temperatures should not be so high as to thermally decompose or fuse the gold-plated subdivided solid. Generally, irradiation temperatures in the range of about 200 to 800 F., preferably about 400 to 700 F., will be employed. Generally the irradiation will be carried out at atmospheric pressure althoughit will be understood that higher or lower pressures. may be employed if desired; 7

The conversion of the naturally-occurring gold (gold 197) to radioactive gold (gold 198) as stated above involves the action of the slow (or thermal) neutrons. The time of irradiation will depend upon the radiation dosage rate available as well as the amount of radioactivity desired in the irradiated product. Generally, irradiation times in the range of about one day to one month, usually about'five to '14 days will "be employed. Generally, for tracing operations the irradiated goldplated subdivided solids should have a radioactivity in the range of about 0.1 to 10, preferably, about 0.5 to 2 millicuries per gram. i i

Although the above-described method wherein the subdivided solid is initially gold plated and then irradiated is particularly preferred, there may be certain instances wherein'other methods of preparation may be preferred. For example, if the subdivided solids contain substantial proportions of metals such as iron, cobalt, barium, strontium and nickel (which themselves become highly radioactive with a longjhal-f life by neutron irradiation), it is preferred to initially irradiate'the reducible compound 55 gold usually'in "an atomic pile and to then plate' th'e subdivided solid withthe compound of'radioactive gold. In this procedure, the same conditions as .ured by close control of injection.

terials which may or may not serve as carriers for plati num, cobalt, molybdenum 'or other active metal catalysts. Such finely divided porous solid catalysts are employed extensively, as is well known, in fluidized hydrocarbon conversion processes such as fluid catalytic cracking, fluid hydroforming and the like. Such fluidized solids are also present in other fluidized hydrocarbon conversion processes such as fluid coking wherein the fluidized solids are finely divided particles of coke. Such subdivided solids as described above may be plated with radioactive gold in accordance with the present invention and employed as a radioactive tracer in the above-described hydrocarbon conversion processes.

For example, radioactive tracer materials of the present invention may be employed, as radioactive tracers in a fluid ca'talytic'cracking process in the following applications: 1

(1) Measuring flow rates in transfer lines. is normally very difficult. With the present tracer, it is possible to. add a small amount to a line and measure the linear velocity by timing the appearance of 'the active zoneat a measured distance downstream. The mass velocity may be determined by bleeding in tracer at a low, measured rate and determining the fraction active taken at a point downstream. Local conditions of reverse flow, such as sometimes occur, may also be meas- These techniques are also applicable to'the flue or stack so that losses to the atmosphere may be measured. This would not be possible with a long half-life isotope for health reasons.

(2)The rate of mixing in a fluid bed may be determined by making an injection of tracer of such particle size'as to not be blown out of the bed, at either top or bottom. A scintillation or other counter placed outside the shell of the vessel at a remote point :will then record the time required for the bed to become uniform in respect to tracer. (3) A more complicated test may be used .to determine the effect of some variable such as the rate of addition of 'fresh catalyst on the rate of stack loss. By adding tracer to the fresh catalyst stream and determining the fraction of tracer in samples drawn from the stack, it is possible to correlate these two variables.

(4) Bypassing of the normal flow path may be detected by timing the appearance of radiation at a point normally sealed off from the injection point. Thus, leaks may be. found in the internal structure of a unit. Conversely, stoppages can be detected by failure of the tracer to take'the path which 'is expected to be the shortest.

(5) Attrition or 'loss of particle sizemay be studied b'y following'the'distribution of activity among the various fractionsfrom a cascade'?or Roller 'particle'size analyser following the injection of a tracer of large particle size.

(6) The total inventory of catalyst may be measured by injecting coarse tracer and, after mixing is complete measuring the activity of an average sample.

One feature of the present invention relates to a novel method of collecting a sample of fluidized, finely divided solids (containing radioactive tracer material of the present invention) from a fluidized solids process. More particularly, the sampling technique comprises drawing a portion of the fluidized, finely divided solids at a point in the fluidized solids system into a steam aspirator and then condensing the steam to thereby obtain a sample of the entrained finely-divided solids in the condensed steam. More particularly, this sampling technique may be carried out by inserting the steam aspirator into the line or vessel on a long probe and withdrawing a steady stream of gas, dust and steam. On condensing the steam, a liquid water stream containing the dust is obtained, which may be continuously scanned with a counter connected to a ratemeter and recorder. Samples of the water stream may also be collected for more detailed study.

The present invention may also be applied to tracing operations in a fluid coking process. More particularly, the same techniques may be followed as in fluid catalytic cracking to determine line flow rates, bed mixing, and leaks or stoppages. The processes are similar to that extent, but differ in that coke is withdrawn instead of fresh catalyst being added and that particle size increases rather than decreases. Thus, an experiment may be conducted to determine the effect of feed stock on withdrawal rate, the latter being practically impossible to measure by ordinary means. Also, the rate of increase of size of particles may be determined by changes in the distribution of activity in various sieve fractions.

The present invention also may be applied to determine the efliciency of grease filtering. More particularly, this may be accomplished by adding to a grease, finely divided particles of dirt which are normally found in the grease and which have been plated with gold 198 in accordance with the present invention, filtering the grease and then measuring the radioactivity of the filtered grease or of the filter screen, or both. Specifically, this tracing application of the present invention permits more complete evaluation of a grease filter than any other known means. Particles of the desired size range may be selected, plated and activated. A commonly desired range is 125-150 microns, which should be removed by any good filter. By this process, such particles may be sifted etc. in a dry state before activation, and checked by microscope measurement before being put in the grease. Filters with irregular or unknown shape are easily evaluated.

The technique is to add a controlled amount of plated and activated particles to the grease and filter it. The particles on the filter may be counted with a small detector or the general level of radiation from the grease before and after filtration may be determined, depending on whether the minimum size retained or maximum size passed is being studied. The particle counting is especially significant if it is suspected that some areas are ineflective as in a stacked plate filter.

The amount of the present radioactive material which is employed as a radioactive tracer should be suflicient to be detected radioactively by conventional radiation detectors. The amount of radioactive tracer added, however, should not be great enough to constitute a radiation hazard to personnel. The particular amount selected for a given application, however, is well within the skill of a person skilled in the art. Generally, When the present radioactive tracer is employed in fluidized solids systems, the amount of radioactive material should be about 0.1 to 1000, preferably, about 1 to 100 microcuries per ton of subdivided solids. Proportions of radioactive tracer material corresponding to the above general concentrations may be employed in other tracing operations. The radiation detectors employed to measure the radioactivity of samples obtained from the process in which the present radioactive materials are employed as radioactive tracers may be any of the well-known conventional radiation detectors. However, radiation detectors particularly adapted to measure gamma rays are particularly preferred. Specific examples of radiation detectors suitable for use in the present invention include scintillation counters with sodium iodide crystals, Geiger tubes and windowless proportional counters. in the art and need not be described further herein. For example, see Nuclear and Radio Chemistry, Friedlander and Kennedy, chapter 8, pages 224 and 2/19 (1955).

The invention will be more fully understood by reference to the following examples. It is pointed out, however, that the examples are given for the.purpose of illustration only and are not to be construed as limiting the scope of the present invention in any way.

Example IPreparati0n of radioactive tracer 72 grams of microspheroidal cracking catalyst (Davidson 3A cracking catalyst (13% alumina, 87% silica)) was separated in a cascade analyzer to give 12 grams of 20-- 40 micron diameter powder. This was mixed with 10 cc. of distilled water containing 0.17 g. AuCl .3H O and 0.25 g. oxalic acid. The mixture was barely wet, the liquid just filling the voids in the catalyst. This was then baked at 25 0 F. to complete the reduction slowly, and at 1000 F. to dry out the powder. The product was pale lavender in color. It was then vigorously shaken and sifted to remove surface gold, leaving about 50 milligrams in the pores of the catalyst, now weighing 11.0 grams. The sample was then subjected to neutron bombardment at the rate of 5 X 10 neutrons/cmfi/sec. for one week, converting part of the gold 197 to gold 198, to the extent of 50 millicuries activity when measured after 24 hours decay. I

The quality of the traced powder was verified by adding 0.36% to standard catalyst and running a cascade analysis; 99.5% of the activity was in the 0-40 micron cut, and 81% in the 20-40 micron cut. Also a small amount was used to make an autoradiograph on No-Screen X-ray film (Eastman). Eight hours exposure (after 9 days decay) showed every particle active.

Example IIUse of radioactive tracer in fluid catalytic cracking process The objective of this test was to determine whether a certain cyclone dust separator inside the regenerator of a fluid catalytic cracking unit was functioning properly in its discharge of dust back to the catalyst bed. It was believed that the discharge valve on the dip-leg might be stuck open or that some other leak might be causing upwards flow rather than the down-flow normally obtained. This would permit, in either case, the discharge of course catalyst from the bed to the duct leading from the regenerator vessel. To perform the test, three injections were necessary:

(1) Into the dip-leg in question (2) Into a dip-leg known to be in good working condition (3) Into the space above the cyclones where coarse catalyst would be forced by a leak in the dip-leg in question These injections were made, each consisting of 10 millicuries of the catalyst whose preparation is described in Example I. The tracer was blown in by a considerable volume of compressed air so that it would rapidly penetrate through the one inch sample line leading into the vessel. A scintillation counter connected to a ratemeter and recorder was placed on the duct leading from the regenerator.

The results were as follows: Tests 1 and 2 showed identical results, a small peak after some delay. Test 3 showed a larger peak of about counts per minute con- Such radiation detectors are well known 7 siderably sooner. It was concluded that the tracer had passed through the bed on Tests 1 and 2 while Test 3 had gone-direct to the duct; Ifience, the dip-leg-inquestionw-as functioning correctly. 1

Example I'IlUse of radioactive tracer in fluid coking process with neutrons. The presence of small amounts ofphosphorus 32 and sulfur 35 in the product do not interfere with its usefulness as they are beta emitters only, .and not to be released in any appreciable :quantity.

The objective is to determine the degree of mixing in the burner, and to correlate it with the rate at which the air is fed. The function of the burner is'to reheat the coke from the reactor so that on returning to the reactor it will continue the cracking reaction. An injection of 200 millicuries (50 grams) :ismade at the bottom of the vessel, using compressedair to blow it out of its container. A scintillation counter is set up near the upper bed surface and a record kept of the count rate until all peaks have disa peared and a uniformly declining reading due to transfer of coke to the reactor sets in. The time for this to happen is the mixing time. The test is then repeated with other air rates until the minimumrate is found to give mixing in a reasonable time.

Example I VUse of radioactive tracer in'grease filtering It was desired to evaluate a laboratory grease filter for ability to remove fine particles. The screen involved was DutchTwill weave, 500 x 28 mesh, so that it is impossible to calculate the equivalent square mesh. Past experience had indicated an equivalent of about '250 .mesh standard sieve.

about 0.001 and 1.0 percent. 1

4. A radioactive tracer according to claim, 1 wherein the radioactivity is .in the range of"0.1 to. ,1-0 millicuries per gram. a

5. A method for preparing a radioactive material useful as a radioactive tracer which comprises mixing a subdivided solid with an aqueous solution of a reducible compound of gold, reducing the reducible compound of gold to thereby plate said subdivided solid with gold metal, and subjecting the gold plated subdivided solid to the action of neutrons .to form radioactive ,gold

6. A method for preparing a radioactive material usefulas a catalyst tracerin a hydrocarbon conversion process which comprises mixing a finely-divided, porous hydrocarbon conversion catalyst with an aqueous solution of gold chloride, heating said mixture under reducing con- 7 ditions to drive .off the water and to plate ,saidcatalyst the radiation was 465 counts per minute above back- 7 ground, a reduction of 18%. Assuming a normal distribution of particle sizes between 20 and 40 microns, this indicates an equivalent poresize of 36 microns.

What is claimed is: 1. A radioactive tracer comprising a silica-aluminahydrocarbon conversion catalyst plated with radioactive gold with gold, and then irradiating the vgold plated catalyst in an atomic pile to form g0ld 7. Method according to claim 6 wherein said hydrocarbon conversion catalyst issilica-alumina.

References CitedinIthe-file of this patent .UNITED STATES PATENTS OTHER REFERENCES Catalysis, Inorganic and Organicfby Suphia Berkman et al., Reinhold Publishing Co.,jN.Y., "1940, pages 439, 440, 442. V

AECD-2566, Feb. 15, 1949, declassified Apr. 18, 1949, pages 1-3. US. Atomic Energy Publication.

Radioisotopes in Industry, by John R. Bradford. Reinhold Publ. Corp, N.Y..( 1953). Pages 299, 300.

Nucleonics, Apr. 1955, pages 18-21. 

1. A RADIOACTIVE TRACER COMPRISING A SILICA-ALUMINA HYDROCARBON CONVERSION CATALYST PLATED WITH RADIOACTIVE GOLD 